Conventional image capturing systems collect rays of light diverging from points in the object space of a lens, and cause the collected rays to substantially converge at one-to-one corresponding points in image space of the lens that is coincident with the plane of an image sensor. Such image capturing systems often include an aperture stop of adjustable size to control the light rays that pass through the lens. The distance to the object, the size of the aperture, along with the focal length of the lens, determines the location and depth of the useful image field, which should include the image plane of the image sensor to obtain a focused image of the scene. If light rays from a point in the scene are focused by the lens at or near the plane of the image sensor, that point is focused with respect to the image sensor. Conversely, if light rays from a point in the scene are focus by the lens substantially in front of or behind the plane of the image sensor, the point may be out of focus with respect to the image sensor.
If the depth of the scene is shallow or the distance to the scene is large, relative to the focal length of the lens, then instances may arise where most of the light collected from points located on objects of interest in the scene will focus at, or near, the image plane. If, however, the distance to the scene is relatively near and objects of interest in the scene exist at substantially different distances from the lens, it may not always be possible to obtain, at the image sensor plane, an optical image wherein all the objects of interest appear, concurrently, in focus. In such instances, parameters of the optical system must be dynamically changed to selectively focus on one aspect of the scene at a time.
A conventional method for adjusting the focus of a lens is to mechanically move rigid lens elements along the optical axis, relative to the position of the image sensor, thereby moving the position of the image plane. However, such methods can be slow, e.g., due to the physical mass of the objects that must be repositioned. Other methods include using a deformable lens with an adjustable curvature. While deformable lenses may be marginally faster than other mechanical techniques, such devices often lack the optical precision, reliability and temperature stability of conventional optical components. Still other methods involve modifying the refractive index of a liquid crystal material by applying a variable voltage potential resistively impeded in such a way as to control the optical power of a lens element. Other known methods seek to focus the image in the digital domain. These methods involve digital de-convolution of the optical point-spread-function in order to restore a focused digital image. While methods of digital focusing can be very fast, image sensor noise and other limitations of the digital image representation cause such methods to lack the absolute focusing range of devices that focus in the optical domain.